When securing rock, e.g. in tunneling or mining draw bolts are used with a tie rod that is anchored in the rock and on whose free end a nut is screwed. The nut rests against an anchor plate, which rests against the rock. In the case of steel draw bolts, the nut is usually tightened with a torque wrench. This clamping process is quite inaccurate because the torque applied with the wrench has a relatively wide range of fluctuation, and the process of setting the anchor plate can reduce the rod tension in an uncontrolled manner.
In the case of draw bolts made of fiber reinforced plastic. this clamping process is inapplicable because the fiber reinforced tie rods have only negligible torsional strength.
A process is known for clamping or prestressing tie rods made of fiber reinforced plastic, as described in U.S. Pat. No. 4,523,880. The free end of the rod has a buttress thread on which a nut is screwed. The nut bears against an anchor plate. To date a device has been used to clamp this anchor, such device having an auxiliary nut which is screwed on the free end of the rod, outside the main nut. The auxiliary nut can be axially moved by a hydraulic cylinder in the frame of the device. The frame has a tripod, which is braced against the rock outside the tie plate. When the cylinder is loaded, the auxiliary nut preliminarily clamps or prestresses the tie rod and the nut can then be tightened against the anchor plate by hand. This process and this device are quite complicated.